Press hardened steel alloys are being used for sheet metal parts incorporated in vehicle body structures that may be assembled together with rivets. One example of a press hardened steel is cold rolled boron steel sold under the designation Docol® 22MnB5. Press hardened steel can be water cooled or oil cooled to desired level of hardness from 450 to 520 HV. Press hardened steel may be annealed to reduce the hardness to 140 HV.
Press hardened steel parts may be assembled to other steel parts by welding. However, new automotive assemblies may include combinations of parts made of different materials such as aluminum and composite parts. A press hardened Ultra High Strength Steel (UHSS) beam and a composite part or an aluminum part cannot be efficiently joined together in a welding operation. The preferred technique for joining such part assemblies is to rivet or otherwise fasten the parts together. The hardness of such high strength parts poses significant challenges in high volume manufacturing operations because the rivets have difficulty penetrating the press hardened UHSS beam.
Press hardened UHSS parts may also be used to manufacture controlled energy absorption applications but the high hardness and tensile strength of the parts prevents substantial energy absorption.
One approach to solving the above problems is to locally soften the part while in the hot stamping die. However, locally softening of the part while disposed in the hot stamping die is unreliable because it is difficult to control all of the process variables in the hot stamping die.
This disclosure is directed to solving the above problems and other problems as summarized below.